Historically, the thermal treatment of thin wall aluminum alloy castings that have been formed in high pressure die cast (HPDC) process is problematic and often results in defective parts and high scrap rates. For example, these types of castings often have complex shapes, surface features, apertures, and variations in their cross-sectional thickness that make it difficult to apply thermal treatments to the castings in a uniform manner. It has been found that unevenly-applied thermal treatments can often create large temperature gradients through the thickness or across the expanse of the alloy material during thermal treatment, resulting in dimensional distortions that remain set within the casting material after the thermal treatments are completed and the casting has returned to an ambient equilibrium temperature. In addition, the thin wall sections of the casting can also be particularly prone to distortion if not properly supported during thermal treatments that raise the temperature of the casting to highly-elevated levels, such as those applied during a solution heat treatment, that soften the alloy material and allow portions of the part to deflect or sag under its own weight. Whether caused by temperature gradients or sagging, if the dimensional distortion of the casting after thermal treatment exceeds predetermined tolerances, the casting is generally scrapped.
Previous attempts to control the sagging created during solution heat treatments include full position fixtures, not shown but known to one of skill in the art, that are tightly or with close tolerances clamped around the castings shortly after their removal from the die, and which then travel with the castings throughout the thermal treatments to rigidly constrain the castings to reduce sagging and other thermal distortions that could pull the metallic parts out of dimensional tolerance. By their very presence, however, the full position fixtures can often impede or block the flow of thermal fluids to portions of the casting material, thereby exacerbating the temperature gradients across the expanse of the part. This can lead to the formation of internal stresses that cause the castings to spring out of shape when the full position fixtures are removed after the thermal treatments are completed.